Fouling in commercial fluidized bed reactors, including gas phase polymerization reactors, is a significant operational issue. Fouling negatively impact operational efficiency and ultimately requires time-consuming shutdown and maintenance.
Understanding the causal factors of fouling within the reactor systems would be beneficial in reducing fouling. Fouling in fluidized bed reactors can be strongly affected by physical processes within the fluidized bed reactor, such as electrostatic charge and solids carryover within a recycle loop.
Commercial probes, including those commonly referred to as static probes and acoustic probes, exist for measuring certain physical parameters within fluidized bed reactors, such as electrostatic charge and solids flow. In common practice, however, these probes do not reliably directly measure these phenomena, and are instead dominated by noise and/or artifacts in the signals. Thus, probe signals have proven to be of limited or no value in monitoring the operational status of or diagnosing inefficiencies in fluidized bed reactors.
Electrostatic charge can affect commercial process units such as chemical reactors, granular particle handling equipment, transfer lines, holding tanks, and shipping containers, for example. The types of operations can include fluidized bed reactors for producing a variety of chemical products such as gas, liquid or solid products such as polyethylene. Cryogenic processes or handling equipment are another notable case where the dry environment can lend itself to electrostatic charge buildup in at least some portions of a process, especially if solids such as ice form due to the cryogenic conditions. The buildup of electrostatic charge on particles, and/or process components results in the formation of an electric field, which then exerts forces on particles or components within a given process or system. Additionally, in cases where the electrostatic charge is sufficient, electrostatic discharge events can occur, which by themselves can be deleterious to reliable or safe operations, or simply an indicator that electrostatic effects are present at a given moment.
For example, commercial polyethylene (PE) reactors utilize a fluidized bed to suspend catalyst particles that grow into PE resin particles by converting ethylene gas into polyethylene resin. Collisions between catalyst particles, resin particles and also the reactor wall can result in the particles becoming charged. The wall can also become charged wherever it has an insulating coating or surface deposit or layer.
If the net charge per volume (ρ) in a cylindrical reactor is uniform, the electric field is given by: E(r)=(ρr)/2∈, where r is the cylinder radius, E(r) is the electric field as a function of reactor vessel radius and ∈ is the relative permittivity of the volume. This electric field is greatest at the reactor wall, and creates a force (F) on the charged particles given by F(r)=qE(r), where q is the particle charge. Both F and E are still a function of radius as mentioned above.
Particles with charge of the same sign as the bulk net charge density experience a force towards the wall. If this force is large enough, it can pin the charged catalyst and resin particles to the wall, and they tend to grow into PE sheets (sheeting) that eventually fall off and clog up the resin discharge system, forcing a shutdown of the reactor. In addition, if the electric field is larger than the Paschen breakdown strength of the gas in the reactor, electrical discharges, or sparks can occur through the gas. Any isolated conductors in the reactor can become charged by particle impact, and they can also spark to nearby metallic objects. In addition, the insulating coating on the reactor wall can charge to a level that supports propagating brush discharges across and through the wall surface.
It is desirable to instrument the reactor with sensors that can indicate a highly charged condition, because that can eventually lead to sheeting and a forced reactor shutdown. Advanced knowledge of a sheeting condition allows operating parameters to be adjusted to eliminate the condition. A highly charged reactor condition can be accompanied by sparking inside the reactor, while lower levels of charge would not result in sparks. Therefore, sparking can be used as an indicator of a highly charged reactor, and indirectly, as a warning that the reactor is in a condition conducive to sheeting.
It is well known that electrical sparks emit electromagnetic waves, typically in the radio frequency (RF) part of the electromagnetic spectrum between about 100 kHz and 10 GHz. Due to the challenging environments encountered in chemical process equipment, especially within a high temperature fluidized bed with reactive gas mixture, no prior art exists for detection of RF signals arising from electrical discharges. In simpler environments, such as assembly rooms for sensitive semiconductor components, some technology does exist. For these simpler environments, the current art includes technology such as the 3M company's EM Aware1, which contains radio frequency receivers with appropriate antennas used to detect sparks by receiving these radio waves. The amplitude, spectral distribution and radiation pattern of the emitted waves depends on the source of the spark. 13M™ EM Aware TNG ESD Event Monitor. Models 3M034-3-TNG, 3M034-030-TNG and 3M034-031-TNG
As indicated in the EM Aware user guide published by the manufacturer, this technology is intended to be used only as follows. “Intended Use”—The 3M EM Aware TNG ESD Event Monitor monitors up to four key parameters that keep you aware of critical symptoms of ESD problems: 1) ESD events; 2) static voltages; 3) ionization balance; and 4) charge decay. The thresholds for these parameters are fully adjustable to suit your needs. The improved design features a metal case module with built-in LCD display, a control joystick, remote antenna, power supply and a data output.
The monitor system must be installed as specified in this user's guide. It is intended for use in the following environmental conditions only: (1) indoor use; (2) altitudes up to 2,000 meters above sea level; (3) temperature range of 10° C. to 40° C.; (4) maximum relative humidity of 80% for temperatures up to 31° C., decreasing linearly to 50% relative humidity at 40° C.; and (5) pollution degree two (office, laboratory, test station).
It would be desirable to have instruments which can reliably measure and monitor electrostatic phenomena within these systems. These instruments could also be combined with new methods for processing and interpreting probe signals in fluidized bed reactor systems. It would further be desirable to have new methods, which may rely upon the use of these instruments and processing methods to provide for more efficient system operation and reliability.